This invention relates to compositions suitable for ink jet printing and to methods in which these compositions are used for printing on substrates.
Ink jet printing methods are well known for application of compositions to a wide range of substrates. Ink jet printing methods are versatile and particularly valuable for printing indicia on substrates used in high speed processes. Examples of such processes include processes of packaging food products in which batch and date information is applied to the packaging. In processes in which the packaging is formed from a polymeric film (e.g. sweet wrappers) the film passes rapidly from the printing station to a position at which the product is wrapped. In such processes it is important that the ink composition is substantially dry before the wrapping process begins, since if it is not then smudging or even removal of the printed indicia can occur. It is also important that the ink has a composition which adheres well to the substrate, since if it does not, in this case also smudging and removal can occur.
Particular problems arise with substrates which are based upon polymers of monomers which include propylene (polypropylenes) or polymers of monomers which include ethylene (polyethylenes). Adherence to these substrates is difficult and is made more difficult when, as is common, they are treated with a release agent (often necessary for subsequent processing of the film to form a product wrapping).
Therefore it would be desirable to be able to provide an ink composition which is capable of rapid drying and is also capable of being formulated to include binder materials which give good adhesion to substrates, in particular the polypropylene- and polyethylene-based substrates mentioned above.
It has been found in the past that solvent components such as methyl ethyl ketone (MEK) and acetone have high drying rate and are thus suitable for use as solvents in compositions which are required to dry rapidly. However, it has been well known for many years that compositions which are formulated to dry rapidly on application to the substrate can give other problems. In particular, rapidly drying ink compositions can lead to problems with blocking of the nozzles, and ink drying in the gutters, of the printing equipment used to apply the ink to the substrate. For this reason inks which include fast drying solvents such as MEK have routinely included humectants and additional solvents known to have higher boiling point, in order to prevent this, which in turn slows down the drying time.
It would be desirable to be able to solve the above mentioned drying and adhesion problems without introducing additional problems of blocking and crusting in the printing equipment. WO92/14794 is concerned with the problem of providing a rapidly drying ink which dissolves commonly-used dyestuffs and is particularly concerned with the problem of replacing the commonly-used solvent MEK. It is also concerned with providing an ink having suitable viscosity. WO92/14794 uses a carrier medium comprising a lower alkyl ester of a lower alkyl carboxylic acid and generally also a lower alkanol. Alkyl moieties for these materials of length C1 to C5 are mentioned generally but the majority of the examples include ethyl acetate, which is the highly preferred alkyl ester for use in WO92/14794. Ethyl butyrate and butyl acetate are also used in other examples.
U.S. Pat. No. 5,990,197 discloses ink formulations generally containing a near infrared fluorophore copolymerised with a polyester, a binder, an organic solvent soluble electrolyte and an organic solvent selected from a C3-C6 aliphatic ketone, a C3-C6 aliphatic ester, a lower aliphatic alcohol or combinations thereof. The example inks use 2-butanone or ethanol or a mixture of the two as the solvent. Methyl acetate is mentioned in a list of C3-C6 aliphatic esters but of these ethyl acetate is said to be preferred.
GB-A-2030075 discloses an ink jet printing process using opaque inks. The ink contains resin or polymer dissolved in a solvent. A wide range of solvents is disclosed, and a list is given reciting aliphatic alcohol, ketones, aldehydes, ethers, esters, glycols, glycol ethers, hydrocarbons and lactones. Methyl acetate is mentioned in a list of ester-type solvents but in the examples the solvents used are methanol or mixtures of methanol with MEK or mixtures of methanol, MEK and acetone.
According to a first aspect of the invention we provide an ink composition suitable for use in an ink jet printer comprising colourant, binder, and solvent which comprises methyl acetate. We also provide, in a second aspect, a method of printing onto a substrate comprising applying an ink composition to the substrate by an ink jet printing method, in which the ink has a composition which comprises colourant, binder, and solvent which comprises methyl acetate.
Surprisingly, we find that the use of methyl acetate as a component of the solvent allows the production of a composition which simultaneously solves the problems mentioned above. The ink dries sufficiently rapidly to prevent smudging and removal of indicia but does not lead to problems of blocking and crusting of printing equipment and also dissolves the materials preferred for obtaining good adhesion to substrates such as polypropylenes and polyethylenes.
Furthermore, we have found that the composition of the invention allows for the use of a wide range of resins and binder polymers. In particular it permits the use of a wider range than would be possible with acetone and MEK.
In addition, we find surprisingly that compositions according to the invention give faster drying time than compositions in which the known fast-drying solvent MEK is used as the only solvent and even faster than some MEK/acetone blends.
In the invention the solvent comprises, as an essential component, methyl acetate. This solvent component has the advantages, discussed above, of fast drying time without inducing nozzle blocking and gutter crusting problems. It is also capable of dissolving a wide range of binder polymers, especially acetal polymers such as polyvinyl butyrals (PVBs), which are particularly beneficial for provision of adherence properties. We have found that other fast-drying solvents such as acetone and MEK do not dissolve PVB binders except under stringent and harsh conditions unsuitable for use in printing processes.
Methyl acetate has the further advantage of a low photochemical ozone creation potential (POCP) of 2.5. POCP is an arbitrary scale of atmospheric chemical activity based on ethylene at 100 and very stable organic materials at zero.
On this basis, we also provide further aspects of the invention as follows.
In a third aspect of the invention we provide an ink composition suitable for use in an ink jet printer comprising colourant, binder and solvent which comprises methyl acetate and from about 4% to about 22%, by weight based on total weight of composition, ethanol.
In a fourth aspect of the invention we provide an ink composition suitable for use in an ink jet printer comprising colourant, binder and solvent which comprises methyl acetate and acetone.
In a fifth aspect of the invention we provide an ink composition suitable for use in an ink jet printer comprising colourant, binder which comprises an acetal polymer and solvent which comprises methyl acetate.
The invention also provides methods of printing onto a substrate comprising applying an ink composition to the substrate by an ink jet printing method using any of the ink compositions of the third to fifth aspects of the invention.
In a further aspect of the invention we provide a method of printing onto a substrate based on polymers selected from the group consisting of polymers of propylene, polymers of ethylene and polymers of propylene and ethylene, comprising applying an ink composition to the substrate by an ink jet printing method, and the ink has a composition which comprises colourant, binder and solvent which comprises methyl acetate.
In a further aspect of the invention we provide a process for making an ink composition of the invention comprising mixing at least part of the binder and at least part of the solvent to form a first pre-mix, separately mixing all remaining components except colourant to form a second pre-mix, combining the first and second pre-mixes and adding to this mixture the colourant.